As a data amount a computer device or the like deals with increases, data transmission by electrical signals has been replaced with the one by optical signals. For example, some transceivers or other various devices and apparatuses transmit or receive data by optical signals. In recent years, fabrication of an optical integrated circuit, in which a transmission unit, a reception unit, and an optical wiring unit of these transceivers or devices and apparatuses are formed on a silicon substrate, has been tried by using a technology referred to as silicon photonics (for example, refer to Non-patent Literature (NPL) 1 “Y. Urino et al., Optics Express, Vol. 19, No. 26, pp. B159-B165, December, 2011”).
On the other hand, as an integrated light source formed on a silicon substrate, a structure in which a semiconductor laser (Laser Diode, abbreviated to LD) array is mounted on a silicon wire waveguide platform by a flip chip mounting method is disclosed. In this method, in order to improve optical coupling between the waveguide and the LD, it is preferable to set the spot size of the waveguide identical to the spot size of the LD, and, in order to increase alignment tolerance, it is preferable to enlarge the spot size. As a structure to achieve these preferences, a structure in which a spot size converter is arranged on an end face of a waveguide connected to an LD is disclosed (for example, refer to Patent Literature (PTL) 1 “International Publication No. 2008/111447”, and NPL 2 “T. Shimizu et al., IEEE 8th International Conference on Group IV photonics, pp. 181-183, September, 2011”).
In NPL 1, after a waveguide section, a modulator section, and a photodetector section are formed, a light source is formed. For an optical integrated circuit, constraints on a manufacturing process become more critical as the process proceeds to later process steps. Therefore, due to constraints on the manufacturing process, a core of a spot size converter, which improves optical coupling with the light source, is formed in a waveguide section process step and a wide tapered structure formed by core layer only is used, causing an increase in coupling loss between an LD, which is a light source, and a waveguide.
On the other hand, in NPL 2, to improve optical coupling between an LD and a waveguide, a spot size converter having a core layer with a square cross-sectional shape is used. As a result, a layer which functions as a core layer for the spot size converter has a thick film thickness, and the core layer causes level differences. In particular, in forming a spot size converter, in order to prevent etching damage to the active sections such as a modulator and a photodetector, it is necessary to use a countermeasure such as forming a passivation layer or an etch stop layer on the active sections whose core layer is removed. Also, in forming electrical wiring sections in active sections such as a modulator and a photodetector after a spot size converter is formed, it is necessary to use a countermeasure to prevent breaking or shortening of the electrical wiring when the electrical wiring crosses upper layer of the core layer, which causes large level differences. In consequence, the manufacturing process has become complicated.